Lifespan of a Photon: Debunking the Myth of Time Travel

The fact that a photon moves only through space (this is what I have read, it can be wrong), not time is said to be the reason why it has so long lifespan, much longer than the calculated lifespan of the universe. Is this wrong? if so, please explain why.

I know that it is impossible to define where a photon really is, but it does move.

rolnor said:

TL;DR Summary: The fact that a photon moves only through space (this is what I have read, it can be wrong), not time, is said to be the reason why it has so long lifespan, much longer than the calculated lifespan of the universe. Is this wrong? if so, please explain why.

The fact that a photon moves only through space (this is what I have read, it can be wrong), not time is said to be the reason why it has so long lifespan, much longer than the calculated lifespan of the universe. Is this wrong? if so, please explain why.

You've posted this in quantum physics, but your question is really more about General Relativity. Light moves through spacetime along a null worldline. That means that locally the speed of light is measured to have an invariant value. It also means that we cannot assign proper time along the light's trajectory: which, being null, is neither timelike nor spacelike.

No great physical or philosophical conclusion can be drawn from this.

To turn to quantum physics.

The photon is the quantum of the quantized electromagnatic field. In the full theory of QED (Quantum Electrondynamics) the photon is a quantum particle - which is a non-classical object - and behaves quantum mechanically. There are states of the electromagnetic field that represent electromagnetic radiation (light waves) and these propagate through vacuum along null worldlines, at an invariant speed, as described above. But, it's not a valid picture to think about EM radiation as a stream of photons moving through space.

Your question, therefore, reveals a confusion between the theories of General relativity, Classical EM and QED. Which makes it not particularly well posed.

Thanx, As understand, than, its only photons that moves this way, along a null wordline? Other particles (wich are QM-objects, they are vibrations in quantum fields) does not move along a null wordline? They move in spacetime along both the time and space-axis?

rolnor said:

Thanx, As understand, than, its only photons that moves this way, along a null wordline?

Light (electromagnetic radiation) moves along a null worldline. The photon is part of the quantum theory of light and can't be described by a classical trajectory. It appears to be impossible to persuade anyone who has learned physics through popular science sources that the classical EM and QED are different theories of light. You may think we have a poor impression of popular science here, and perhaps that's true. But, you can see the problem it causes to anyone who subsequently wants to ask deeper questions. The false and confused pictures presented by popular science have to be unlearned if you want to take the next step. And, in my experience, most people are extremely reluctant to abandon what they have learned from popular sources.

rolnor said:

Other particles (wich are QM-objects, they are vibrations in quantum fields) does not move along a null wordline?

Massive particle move along timelike worldlines.

rolnor said:

They move in spacetime?

Everything moves through spacetime: you, me, the Earth, electrons, light. Your assumption that light does not move through spacetime is wrong.

PeroK said:

Light (electromagnetic radiation) moves along a null worldline. The photon is part of the quantum theory of light and can't be described by a classical trajectory. It appears to be impossible to persuade anyone who has learned physics through popular science sources that the classical EM and QED are different theories of light. You may think we have a poor impression of popular science here, and perhaps that's true. But, you can see the problem it causes to anyone who subsequently wants to ask deeper questions. The false and confused pictures presented by popular science have to be unlearned if you want to take the next step. And, in my experience, most people are extremely reluctant to abandon what they have learned from popular sources.

Massive particle move along timelike worldlines.

Everything moves through spacetime: you, me, the Earth, electrons, light. Your assumption that light does not move through spacetime is wrong.

Thanx, I am trying to re-learn, I really am. If we go back to my initial question, is the long lifespan of a photon a result of the fact that it moves along a null wordline? Of course you hate pop-science, I hate it to, thats why I try to learn how thing really are and I have learnt tons from you nice people

rolnor said:

Thanx, I am trying to re-learn, I really am. If we go back to my initial question, is the long lifespan of a photon a result of the fact that it moves along a null wordline?

No. The question doesn't make sense. The photon is a stable particle, as far as the standard model of particle physics is concerned. As are the electron and the neutrino.

rolnor said:

Of course you hate pop-science, I hate it to, thats why I try to learn how thing really are and I have learnt tons from you nice people

The only real alternative is to start studying from an undergraduate textbook. That said, Feynman's Strange Theory of Light and Matter is a possible starting point for QED.

Thanx, I will look that up. But I think you are wrong, there is possible for a photon to decay, I have seen such calculations. It takes much longer time than the lifespan of the universe. I am not sure if this always happens but it can decay.

rolnor said:

Thanx, I will look that up. But I think you are wrong,

You need to be precise in science. I said explicitly the standard model of particle physics.

rolnor said:

there is possible for a photon to decay,

There may be a possibility, but in the standard model they do not decay.

rolnor said:

I have seen such calculations. It takes much longer time than the lifespan of the universe. I am not sure if this always happens but it can decay.

You may have seen calculations to estimate a minimum lifetime. We can never know for sure that something has an infinite lifetime.

rolnor said:

Thanx, I will look that up. But I think you are wrong, there is possible for a photon to decay, I have seen such calculations. It takes much longer time than the lifespan of the universe. I am not sure if this always happens but it can decay.

PeroK said:

You need to be precise in science. I said explicitly the standard model of particle physics.

There may be a possibility, but in the standard model they do not decay.

You may have seen calculations to estimate a minimum lifetime. We can never know for sure that something has an infinite lifetime.

Here is a paper by Julian Heeck, the lifespan is about 1 quintillion years according to his calculations. Maybe this is not the standard model and I dont know the quality of this Journal. https://arxiv.org/pdf/1304.2821.pdf

rolnor said:

Here is a paper by Julian Heeck, the lifespan is about 1 quintillion years according to his calculations. Maybe this is not the standard model and I dont know the quality of this Journal. https://arxiv.org/pdf/1304.2821.pdf

This estimate of the photon lifetime is predicated on it having a non-zero mass of ##10^{-18}\text{ eV}## (the current experimental upper limit). So just be careful not to confuse the two arguments you've made in this thread because they're contradictory:

• Photons travel on null worldlines ##\Rightarrow## photons are massless ##\Rightarrow## photons cannot decay
• Photons can decay ##\Rightarrow## photons have a non-zero mass ##\Rightarrow## photons travel on time-like worldlines

Thanx, very interesting, I do understand the conflict betveen this statements. Do you believe that a photon has mass?

rolnor said:

Thanx, very interesting, I do understand the conflict betveen this statements. Do you believe that a photon has mass?

You have, I think, misinterpreted that paper in exactly the way that @PeroK warned you against in post #9 above - mistaking a lower bound for a value. It’s not saying that the lifetime is finite. It’s saying that there’s no reason to think it’s finite, but in the unlikely event that it is not infinite it will still be greater than megabazillions of years.

rolnor said:

TL;DR Summary: The fact that a photon moves only through space (this is what I have read, it can be wrong), not time, is said to be the reason why it has so long lifespan, much longer than the calculated lifespan of the universe. Is this wrong? if so, please explain why.

The fact that a photon moves only through space (this is what I have read, it can be wrong), not time is said to be the reason why it has so long lifespan, much longer than the calculated lifespan of the universe. Is this wrong? if so, please explain why.

Yes this is wrong. If that were the reason that it has a long lifetime then it would imply that particles that do “move through time” would have short lifetimes. The electron is a counter example.

The actual reason is that there is nothing else for it to decay into that conserves all of the various conservation laws.

PeroK said:

Light moves through spacetime along a null worldline…..But, it's not a valid picture to think about EM radiation as a stream of photons moving through space.

rolnor said:

Thanx, As understand, than, its only photons that moves this way, along a null wordline?

No. Light moves on null worldlines, is what @PeroK said. Photons don’t move it all, at least not the way you’re thinking of ‘move’.

The problem here is that photons aren’t what you’re thinking when you hear phrases like “quantum of light” or “massless particle”. There’s no really good math-free way of saying what a photon is, but here is the best I can come up with:

We have electrical and magnetic fields (and sometimes these fields form traveling waves which we call “light” and which propagate at speed ##c## in a vacuum). We find that whenever these fields interact with matter, the interaction always exchanges energy and momentum in discrete amounts and at a single point (unlike classical wave behavior, where it is spread out across the entire wavefront). When this happens, we say “a photon was detected” at that point.

Now the idea of photons “moving” makes no sense. A light source transfers energy to the electromagnetic field in units of photons. This changes the field, and at some later time the field will exchange energy with whatever the light source is illuminating, again in units of photons. But that’s not a photon moving from one point to another, it’s two separate interactions with the quantized electromagnetic field at two different locations.

rolnor said:

Thanx, As understand, than, its only photons that moves this way, along a null wordline? Other particles (wich are QM-objects, they are vibrations in quantum fields) does not move along a null wordline? They move in spacetime along both the time and space-axis?

First of all forget about photons being a particle. Before you can understand photons at all you need a good understanding of classical electrodynamics and electromagnetic waves (including light). Then you can learn QED.

The meaning of "photon" in the introducory general-relativity literature is, however (and fortunately for the reader), not about the true modern photon concept, which would involve the very complicated topic of quantum field theory in curved spacetime, which is even more complicated than standard QFT in Minkowski spactime, i.e., special relativity, which neglects the gravitational interaction.

What's really behind the treatment of light propagation in general-relativistic spacetimes is the socalled eikonal approximation, which is the right mathematical tool to derive "geometric optics" from "wave optics" (or rather Maxwell's classical electrodynamics). Here's a introduction:

https://itp.uni-frankfurt.de/~hees/pf-faq/gr-edyn.pdf

I think this is the first time I have seen the word "eikonal" in a B-level thread.

Vanadium 50 said:

I think this is the first time I have seen the word "eikonal" in a B-level thread.

But the eikonal equation and related topics (characteristics, 1st order pde, weak solutions, …) are intuitive and useful in many contexts. Ever heard of the fast-marching algorithm, which can be used for as diverse things as motion planning or the modeling of photoresist development?

I didn't say it wasn't useful. Just that it was the first time I saw it in a B-level (high school) thread,

Vanadium 50 said:

Just that it was the first time I saw it in a B-level (high school) thread,

Based on the OP's responses, the thread has gone through A-->I-->B transitions.

rolnor said:

The fact that a photon moves only through space (this is what I have read, it can be wrong), not time

It is indeed wrong. Unfortunately, it is a fairly common misstatement in pop science sources, even ones by physicists like Brian Greene who should know better.

Vanadium 50 said:

I think this is the first time I have seen the word "eikonal" in a B-level thread.

That's indeed a pity. How can you understand geometrical optics without the eikonal approximation? It's clearly undergraduate material, well posed in a B-level thread!

vanhees71 said:

undergraduate material, well posed in a B-level thread!

Undergraduate is "I" level, not "B" level.

berkeman said:

Based on the OP's responses, the thread has gone through A-->I-->B transitions.

PeterDonis said:

Undergraduate is "I" level, not "B" level.

Harmonic oscillation in thread prefix level. I'm getting dizzy...

Nugatory said:

No. Light moves on null worldlines, is what @PeroK said. Photons don’t move it all, at least not the way you’re thinking of ‘move’.

The problem here is that photons aren’t what you’re thinking when you hear phrases like “quantum of light” or “massless particle”. There’s no really good math-free way of saying what a photon is, but here is the best I can come up with:

We have electrical and magnetic fields (and sometimes these fields form traveling waves which we call “light” and which propagate at speed ##c## in a vacuum). We find that whenever these fields interact with matter, the interaction always exchanges energy and momentum in discrete amounts and at a single point (unlike classical wave behavior, where it is spread out across the entire wavefront). When this happens, we say “a photon was detected” at that point.

Now the idea of photons “moving” makes no sense. A light source transfers energy to the electromagnetic field in units of photons. This changes the field, and at some later time the field will exchange energy with whatever the light source is illuminating, again in units of photons. But that’s not a photon moving from one point to another, it’s two separate interactions with the quantized electromagnetic field at two different locations.

Thanx, is this true for "particles" like electrons and quarks as well? They are vibrations in quantum fields?

berkeman said:

Based on the OP's responses, the thread has gone through A-->I-->B transitions.

This is bullying, not nessecary and not science

rolnor said:

Thanx, is this true for "particles" like electrons and quarks as well? They are vibrations in quantum fields?

Electrons can be described by non relativistic QM. Photons require QED, and quarks require QCD. Even a classical treatment of an electron is sufficient in some applications. There's no classical theory for photons or quarks.

One problem is that you are mixing up concepts from different theories. An I-level thread would focus on undergraduate physics. And an A-level thread implies at least master's level material.

In either case, there would be a solid mathematical basis for discussion.

Another problem is that without this mathematical basis, we are left with a B-level thread on QFT, which makes things difficult.

Of course, in relativistic physics electrons are described by relativistic QFT, i.e., as a quantized Dirac field.

Classical point particles are very strange objects in relativistic physics. Even in classical electrodynamics, described in Minkowski space (special relativity) up to today nobody has been able to find a fully self-consistent dynamics between classical point particles and the electromagnetic field. The best one can do is the first-order perturbative correction to neglecting the radiation reaction all together, the socalled Landau-Lifshitz approximation to the Lorentz-Abraham-Dirac equation, which has severe problems with causality. In classical physics what works is a continuum description of matter together with the electromagnetic field. In relativistic QFT you can consistently define everything at any order in perturbation theory.

In relativistic QFT a particle is a asymptotic free "single-particle" Fock state, i.e., a particle interpretation is only possible in regions, where all "particles" are far away from each other such that you can neglect their mutual interaction and detect them as "free particles".

Even this pretty restricted sense of a "particle notion" in the sense of asymptotic free Fock states is, however, too naive as soon as massless fields are present as in QED, where the em. field is a massless gauge field. There the asymptotic states of (massive) charged particles is not the naive plane-wave mode since due to the masslessness of the photon the asymptotic states are still interacting, because the em. field only goes with ##1/r## with ##r## the distance between particles. The asymptotic free charged-particle state in QED is a so-called "infraparticle". In a hand-waving way you can describe it as a naive asymptotic free charged-particle state surrounded by a coherent state or a "soft-photon cloud". That's among the more advanced topics of relativistic QFT though.

PeroK said:

Electrons can be described by non relativistic QM. Photons require QED, and quarks require QCD. Even a classical treatment of an electron is sufficient in some applications. There's no classical theory for photons or quarks.

One problem is that you are mixing up concepts from different theories. An I-level thread would focus on undergraduate physics. And an A-level thread implies at least master's level material.

In either case, there would be a solid mathematical basis for discussion.

Another problem is that without this mathematical basis, we are left with a B-level thread on QFT, which makes things difficult.

Yes, I know my questions are ignorant, but I do learn from this discussion. I will get some reading. Thanx

It's not ignorance! It's just that relativistic QFT is a quite difficult subject, and it's impossible to understand it without the adequate math. In addition it's also not fully established from a strict mathematical point of view. It needs time to learn all this. For me the most concise introductory treatment is given in

S. Coleman, Lectures of Sidney Coleman on Quantum Field
Theory, World Scientific Publishing Co. Pte. Ltd., Hackensack
(2018), https://doi.org/10.1142/9371

Large parts of this book is also freely available from the arXiv:

https://arxiv.org/abs/1110.5013

More details, particularly the emphasis of the importance of the microcausality constraints can be found in the 3-volume textbook series by Weinberg, Quantum Theory of Fields.

Complementary to that is

A. Duncan, The conceptual framework of quantum field
theory, Oxford University Press, Oxford (2012).

rolnor said:

This is bullying, not nessecary and not science

No, no, it's not meant as bullying at all. Over the years at PF we have come to understand how important it is to tune the level of responses to the level of understanding of the OP asking the question. That is why in many of our technical PF forums we use the prefix system on the thread title, to better help responders know how to respond to the OP's question.

rolnor said:

This is bullying, not nessecary and not science

OK, so what level do you want an answer? Graduate level texts have been suggested. Is this an adequate answer? If not, what do you want instead?

Lack of mental telepathy is not the same as bullying.

I am grateful for all answers that help me understand this matter. One comment seemed unecessary. Maybe I missinterpreted the intention.

vanhees71 said:

It's not ignorance! It's just that relativistic QFT is a quite difficult subject, and it's impossible to understand it without the adequate math. In addition it's also not fully established from a strict mathematical point of view. It needs time to learn all this. For me the most concise introductory treatment is given in

S. Coleman, Lectures of Sidney Coleman on Quantum Field
Theory, World Scientific Publishing Co. Pte. Ltd., Hackensack
(2018), https://doi.org/10.1142/9371

Large parts of this book is also freely available from the arXiv:

https://arxiv.org/abs/1110.5013

More details, particularly the emphasis of the importance of the microcausality constraints can be found in the 3-volume textbook series by Weinberg, Quantum Theory of Fields.

Complementary to that is

A. Duncan, The conceptual framework of quantum field
theory, Oxford University Press, Oxford (2012).

Thanx a lot now I have some reading to do.